The present invention relates to a thin film capacitor, more particularly, an .alpha.-tantalum (Ta) thin film capacitor, and a process for producing the same.
A thin film capacitor is used in hybrid ICs and has advantages over a conventional chip capacitor in the point that the thin film capacitor is produced by a photolithographic pattern generation technique carried out simultaneously with the formation of resistors.
Conventionally, a thin film capacitor of a hybrid IC is produced using .beta.-tantalum by the following procedure. A ceramic substrate, for example, an alumina substrate, is thinly coated by a glass material to fabricate a glazed substrate having a high surface flatness. A thermal oxidation film of tantalum (Ta.sub.2 O.sub.5) is formed on the glazed substrate, a lower electrode consisting of a .beta.-tantalum thin film is formed on the tantalum thermal oxidation film and the upper part of the .beta.-tantalum thin film is subjected to an anodic oxidation, thereby forming a chemical conversion film which is the dielectric layer of the capacitor. Subsequently, an upper electrode is provided on the chemical conversion film by firstly depositing, for example, a nickel-chromium layer, as an underlying layer and then a gold layer. The so produced .beta.-tantalum thin film capacitor has a high reliability and a long life. In the hybrid ICs, in which the thin film capacitors and the thin film resistors are mounted on a single substrate, the substrate is usually selectively glazed on the parts thereof where the thin film capacitors are mounted. However, such a substrate is expensive as compared with an entirely glazed substrate and a non-glazed substrate.
It is reported by L. G. Feinstein et al, in Thin Solid Films, Vol. 20 (1974), page 103, that .beta.-tantalum is metastable and the properties of .beta.-tantalum are greatly dependent on the sputtering atmosphere, in which the .beta.-tantalum is formed. It is pointed out by R. D. Huttemann et al, in IEEE Trans. PHP-11 (1975), page 67, that .beta.-tantalum involves a difficulty in the strict controlling of the film structure and exhibits inferior characteristics of the structure-sensitive properties, such as a leakage current. Typical properties of the .beta.-tantalum thin film capacitor using the chemical conversion film of .beta.-tantalum as the dielectric layer are only 50 V or lower of breakdown voltage and from 2 to a few tens A/F of leakage current after the application of 50 V for 1 minute.
It is proposed by K. Kumagai et al, in Proc. ECC 1973, IEEE, New York, page 257, to eliminate the disadvantages of high leakage current and low breakdown voltage by using N.sub.2 doped .beta.-tantalum instead of .beta.-tantalum. With such use the leakage current can be reduced from the value in the range of 2 A/F at the minimum to 0.05 A/F after the application of 50 V for 1 minute, and the breakdown voltage can be increased from 50 V at the maximum to approximately 120 V.
The present inventors investigated the .alpha.-tantalum thin film capacitor and discovered that, when .alpha.-tantalum is deposited on a polycrystalline alumina substrate by sputtering, the nucleation of the .alpha.-tantalum film during the initial growth stage thereof is non-uniform. A number of defects in the form of a slit or vacancy are formed in the chemical conversion film of the .alpha.-tantalum film mentioned above, and the leakage current and the breakdown voltage are deteriorated by such defects, according to the discovery of the present inventors.